I have heard of someone in the past converting a CB77 into a type II by re-pressing the crank and modding the cam to have both pistons fire simultaneously.
This should be very easy to do on a stock Type II....just rotate 1/2 of the cam 180 degrees?
Surely someone has tried it?
Type II "Twingle"?Type II "Twingle"?66 dream, 78 cb750k, 02fz1, 09 wing
Hi,
Yes, it can be done, I did it the other way around. I had a NOS stock C77 crank and converted this in a NOS CB72 crank. But it's more work then you think, you have to press one half of the crank in pieces, and then build it up again, piece by piece. You cannot just split it and then turn 180 degrees, or turn it without splitting at all. If I had a choice of doing this all over again, I wouldn't do it. It took me a long week to get the crank within spec again. At the moment I'm converting a very good used CL77 crank in a CB72 crank by adding a oil bore to it, that's easy compared with converting a crank from 180 to a 360 degrees, or the other way around. But feel free to try, if you have time on your hands and the machines it can be done. have fun, (and be patient), Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen,
If I already have a 360 degree crank on a C77, and just want to fire both the pistons at the same time, effectively making it a "single", wouldn't you just have to rotate half of the cam? I have a relative with a full machine shop, but shouldn't need it if the cam has an even number of splines in the middle joint. 66 dream, 78 cb750k, 02fz1, 09 wing
Ooops,
Reading is an art isn't it ? No, I can't give you answer on that question, but if the cam is symetrical (even splines), grinding off the center spline would be sufficient. Why would you build a "big bang" engine by the way ? I would love to hear the sound of it for sure, Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Hi,
Back around 1962 in Japan we converted a CB72 Type II into basically a 250 thumper for scramble racing on the hilly short straightaway tracks, had a lot of torque. I don't remember exactly all we did but I believe we just cut the notch off the one cam that slides into the slot on the other half of the cam. Rotate one half 180 degrees, reinsert and lock it down. Its been a long time but I think thats all we did to it. Now both cylinders fire on the same stroke. Don't remember how long we run it but I don't believe we ever had any problems with it. CliffC More torque! I can remember lugging around on my Triumph 200cc Cub years ago and it just felt like it had so much more torque than the Superhawk I traded it for. I checked the specs on newer 250cc bikes now in production, for comparison: Yamaha 250cc twin...18HP.......14 ft-lbs torque Honda 250cc twin.....16 HP.....12.4 ft-lbs torque Suzuki 250 single......17HP......24.9 ft-lbs torque Quite a difference! 66 dream, 78 cb750k, 02fz1, 09 wing
Hi,
A CB72 has a stroke of 54 mm, same as the CB77, the bore is 54 mm for a CB72 and 60 mm for a CB77. The CB72 his exactly square, the CB77 is even more square. These numbers mean that these engines are made for.... reving, the cb 77 with a ratio of 1.11 even more then the CB72 with a ratio of 1.00. But I have to admit, the feeling of torque at low revs will be better with a "big bang" engine.
Last edited by jensen on Wed Nov 25, 2009 2:47 am, edited 1 time in total.
assembly of Japanese motorcycles requires great peace of mind (Pirsig)
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